The present invention relates to an automatic frequency correction apparatus and method for a radio calling system and, more particularly, to an automatic frequency correction apparatus and method which can be suitably used for a mobile station as a constituent of a radio calling system.
Some conventional radio calling systems of this type have automatic frequency correction apparatuses installed in mobile stations. For example, Japanese Patent Laid-Open No. 3-70335 discloses an automatic frequency correction apparatus installed in a mobile station. The automatic frequency correction apparatus described in this reference detects the error between a reception frequency and a local oscillation frequency in a specific interval of reception data from a transmitting base station, and selects correction data corresponding to the detected error, thereby controlling a reference oscillator. With this operation, the reference oscillator is matched with the reception frequency from the transmitting base station to allow the mobile station to perform accurate, stable reception and transmission control.
In a radio calling system of this type, however, to make the local oscillation frequency accurate and stable, each mobile station must use a high-precision oscillator such as a TCXO (Temperature Compensated X'tal Oscillator) and an automatic frequency correction circuit including special components such as a frequency meter and an A/D converter. For this reason, to improve the precision of the local oscillation frequency in each mobile station, the cost and the size of a control circuit inevitably increase, resulting in difficulty in satisfying demands for economical, small-size apparatuses.
In addition, since automatic frequency correction is executed on the basis of the reception frequency from a transmitting base station, a stable reception frequency cannot be obtained in a situation in which the reception ratio of reception signals is low. If a high-precision oscillator is used for this situation, a problem is posed in terms of cost performance.
Furthermore, when the reception ratio of reception signals is low, an automatic frequency correction section cannot execute a stable operation.